What kind of bike are you riding? That would be helpful to know as some may have something similar and can help you with specifics to commuting. I am not familiar with Seattle but you can buy a lot of bike stuff online, especially Amazon.

As far as lights, you're best off having a front and rear. I use Planet Bike lights similar to these: Planet Bike Lights

I don't use a vest, but I am sure you could find one on Amazon as well

Do you have a rack? If not, I really like the Topeak Explorer: Topeak Explorer Rack

Topeak makes an assortment of bags for their racks that attach securely.

I don't think there's anything commuter specific that I carry that would be different from any other kind of cycling. You should have a pump, patch kit and basic tools in case you need to repair something of patch a flat tire. My commuter bike has panniers and I just kind of end up throwing all kinds of junk in them and clean it out over time.

You'll probably get a ton of responses that vary a lot. The cool thing about commuting is that you can pretty much use what works for you. Something that works great for one person may not be of any interest to somebody else.

The key is to just have fun.